Metering Pump

ABSTRACT

A metering pump includes at least a first piston and a second piston which are displaceable in a first cylinder bore and in a second cylinder bore, respectively, the at least two pistons having a common driving device, and the second piston being displaceably connected to and displaced with the first piston for a portion of the stroke length of the first piston.

This invention relates to a metering pump. More particularly, it relates to a metering pump including at least a first piston and a second piston which are displaceable in a first cylinder bore and a second cylinder bore, respectively, the at least two pistons having a common driving device.

Metering pumps are known in a variety of designs, for example in the form of piston pumps in which the stroke length of the piston is often adjustable for controlling the dosing rate.

When two or more liquids are to be mixed in a given proportion, it is common to use two metering pumps, in which the dosing rate may be set at the desired ratio. Examples of liquids that can be mixed are ethanol and a vegetable oil to make up a bio fuel mixture.

It is also known to use piston pumps that have a piston with a relatively large diameter and a piston with a relatively small diameter. The pistons are driven by a common driving unit and have the same stroke length. The dosing ratio is determined by the relative relationship between the areas of the pistons and is thereby fixed. Patent publication US 2007/0286745 discloses an example of such a metering pump in which two dosing pistons are connected to a driving piston and have a common stroke length.

The invention has for its object to remedy or reduce at least one of the drawbacks of the prior art.

The object is achieved according to the invention through the features that are specified in the description below and in the claims that follow.

A metering pump is provided, including at least a first piston and a second piston which are displaceable in a first cylinder bore and in a second cylinder bore, respectively, and the at least two pistons have a common driving device, and the metering pump is characterized by the second piston being displaceably connected to and being displaced with the first piston for a portion of the stroke length of the first piston.

Both cylinder bores are provided with an inlet valve and an outlet valve each. In their simplest form, these may be automatically controlled check valves, but more complicated, controlled valves may be appropriate in some cases.

In one embodiment of the metering pump, the second piston may have a fixed stroke length. In this embodiment, it is, thus, the stroke length of the first piston in the discharge direction that is adjustable and that regulates a dosing ratio of a first liquid to a second liquid.

However, it is possible to adjust how far the first piston will return in the intake direction, in order thereby to alter the stroke length of the second piston. The stroke length of the second piston is thus adjustable. See a more detailed explanation in the characterizing part of the description.

It is possible to limit the stroke length of the first piston so that both pistons will have the same stroke length. It is possible to set the stroke length of the first piston to be so short that the second piston will not be displaced.

The second piston may be displaced in the discharge direction by a first spring which is connected to the first piston. In the intake direction, the second piston may be displaced by a second spring which preferably spans between the second piston and a pump housing.

The first spring is typically formed with a higher spring constant than the second spring and is thereby dimensioned to exert a greater force than the second spring. As long as the first spring pushes against the second piston, the second piston will be displaced in the discharge direction or rest against the pump housing. When the first piston is displaced in its intake direction and the first spring is extended to its free length, the second piston is displaced in its intake direction by the second spring. The free length of the first spring can thus limit the stroke length of the second piston.

The first piston and the second piston may have a common longitudinal axis. In this preferred embodiment, the second piston is formed as an annular piston displaceable on the first piston.

In an alternative embodiment, the stroke length of the second piston in the discharge direction is limited by a stop on the first piston. The second spring is then superfluous as the stop will hit and carry the second piston along in the intake direction.

The device according to the invention provides a metering pump for at least two liquids in which the dosing ratio can easily be changed, even during operation, and in which one driving unit is sufficient to operate both pistons. Compared with prior-art metering pumps, in which two or more liquids are to be mixed, the device exhibits a substantial simplification.

In what follows, examples of preferred embodiments are described, which are visualized in the accompanying drawings, in which:

FIG. 1 shows a metering pump according to the invention as both a first piston and a second piston are in their respective starting positions, from which the discharge stroke starts;

FIG. 2 shows the same as FIG. 1, but here both pistons are being displaced in their discharge strokes;

FIG. 3 shows the same as FIG. 2, but here the second piston has reached its end position in the discharge stroke;

FIG. 4 shows the same as FIG. 3, but here the first piston continues its discharge stroke while the second piston is stationary; and

FIG. 5 shows the metering pump in an alternative exemplary embodiment in which the metering pump has been adapted for operation by a driving device with a fixed stroke length.

In the drawings, the reference numeral 1 indicates a metering pump including a pump housing 2, a first piston 4 and a second piston 6. The second piston 6, which constitutes an annular piston, encircles the first piston 4. Both are displaceable along a common longitudinal axis 8 in the pump housing 2.

The first piston 4 is sealingly positioned in a first cylinder bore 10 in the pump housing 2. The second piston 6 is sealingly positioned in a second cylinder bore 12 in the pump housing 2. The second cylinder bore 12 has a larger diameter than the first cylinder bore 10. Both pistons 4 and 6 are single-acting. Piston seals necessary per se are not shown.

A first cylinder chamber 14 is formed in the first cylinder bore 10 between the first piston 4 and an end cap 16, whereas a second cylinder chamber 18 is formed in the second cylinder bore 12 around the first piston 4, between the second piston 6 and a shoulder 20 in the pump housing 2.

A first spring 22 is in the second cylinder bore 12 between the second piston 6, on the side facing away from the second cylinder chamber 18, and a flange 24 on the first piston 4. A second spring 26 is in a spring bore 19 which forms a widening in the first cylinder bore 10 between the second piston 6 on the opposite side to the first spring 22 and a spring shoulder 28 in the pump housing 2. The first spring 22 has a higher spring constant than the second spring 26.

The first piston 4 and thereby the second piston 6 as well are driven in a reciprocating motion by a driving device 30 which preferably has an adjustable stroke length and adjustable turning positions. The driving device 30 may be, for example, mechanically, hydraulically, pneumatically or electrically operated. The driving device 30 performs a discharge stroke when the piston 4 is displaced in the cylinder bore 10 towards the end cap 16, and the driving device 30 performs an intake stroke when the piston 4 is displaced in the opposite direction to the discharge stroke.

The first cylinder chamber 14 is supplied with a first liquid via a first inflow valve 32 while the second cylinder chamber 18 is supplied with a second liquid via a second inflow valve 34. The first and second inflow valves 32, 34 are connected to their respective liquid containers, not shown

Correspondingly, the first liquid flows out of the first cylinder chamber 14 via a first outflow valve 36 and from the second cylinder chamber 18 via a second outflow valve 38. In this exemplary embodiment shown, the outflow valves 36, 38 open into a mixing vessel 40. Both the inflow valves 32, 34 and the outflow valves 36, 38 consist of check valves here. The first liquid and the second liquid in the mixing vessel 40 may be carried through a static mixer (not shown).

A first pump volume which is in the first cylinder chamber 14 is determined by the stroke length of the first piston 4. The stroke length is determined by the relative turning positions of the first piston 4 in the pump housing 2.

A second pump volume which coincides with the second cylinder chamber 18, by the very fact of the end position of the discharge stroke of the second piston 6 being determined by an abutment against the shoulder 20 and thereby being fixed, is determined by the turning position of the second piston 6 relative to the pump housing 2 before the discharge stroke takes place. The turning position of the piston 6 is determined by the turning position of the piston 4.

In FIG. 1, both cylinder chambers 14, 18 are filled with liquid and the discharge stroke is started. While both the pistons 4, 6 are being displaced as it is shown in FIG. 2, the first liquid and the second liquid are dosed in this part of the discharge stroke in a ratio that is given by the piston area of the first piston 4 relative to the piston area of the second piston 6.

When the second piston 6 hits the shoulder 20, as it is shown in FIG. 3, the liquid flow of the second liquid from the second cylinder chamber 18 stops, whereas the liquid flow of the first liquid continues from the first cylinder chamber 14 as long as the first piston 4 is being moved in the discharge direction. The first spring 22 is being compressed at the same time.

The final mixture ratio of the first liquid to the second liquid is determined by the ratio of the first pump volume to the second pump volume.

During the intake stroke, the second piston 6 is stationary until the first spring 22 exhibits less force than the second spring 26, typically when the first spring 22 has reached its starting length. Then the second spring 26 displaces the second piston 6 during the intake stroke.

Different settings of the discharge stroke could give a substantial range of adjustment for the mixing ratio. By stopping the discharge stroke of the first piston 4 when the second piston 6 hits the shoulder 20, a constant mixing ratio of the first liquid to the second liquid can be delivered all the time. It is also conceivable that the entire discharge stroke of the first piston 4 takes place within a range in which the second piston 6 is in abutment against the shoulder 20. In this case, only the first liquid is dosed.

In an alternative embodiment, the first piston 4 is provided with a stop 42 which is arranged to restrict the travelling of the second piston 6 on the first piston 4. The stop 42 is indicated in a broken line in FIG. 1. The second spring 26 is superfluous here, as the stop 42 strikes against and displaces the second piston 6 in its intake stroke.

In a further alternative embodiment, see FIG. 5, the metering pump 1 is adapted for operation by a driving device 30 with a fixed stroke length, for example a motor with a crank axle and connecting rod, not shown.

In this embodiment, part of the flange 24 projects through axial slots 44 in the pump housing 2. Externally, the pump housing 2 is provided with an axially adjustable, stroke-restricting ring 46 against which the flange 24 is arranged to strike.

The driving device 30 is formed with a spring support 48 which is connected to the first piston 4 via a third spring 50.

A return dog 52 on the driving device 30 is arranged to move the first piston 4 in the intake stroke as the driving device 30 performs its return stroke.

The metering pump 1 according to this embodiment works in the same way as described above, but with the difference that when the flange 24 hits the stroke-restricting ring 46, the discharge stroke is stopped even though the driving device 30 has not gone through its entire stroke length. The spring support 48 thereby compresses the third spring 50 during the remaining discharge stroke of the driving device 30.

When the return stroke of the driving device 30 starts, the third spring 50, the first spring 22 and the second spring 26 are extended, in that order, after which any remaining length of the intake stroke is effected by the return dog 52 carrying the first piston 4 along. 

1. A metering pump comprising at least a first piston and a second piston which are displaceable in a first cylinder bore and in a second cylinder bore, respectively, the at least two pistons having a common driving device, wherein the second piston is displaceably connected to and is displaced with the first piston for a portion of the stroke length of the first piston.
 2. The metering pump in accordance with claim 1, wherein the second piston is moved in a discharge direction by a first spring which is connected to the first piston.
 3. The metering pump in accordance with claim 1, wherein the second piston is displaced in an intake direction by a second spring.
 4. The metering pump in accordance with claim 2, wherein the first spring is dimensioned to exert a greater force than the second spring.
 5. The metering pump in accordance with claim 2, wherein the free length of the first spring restricts the stroke length of the second piston.
 6. The metering pump in accordance with claim 1, wherein the stroke length of the second piston is restricted by a stop on the first piston.
 7. The metering pump in accordance with claim 1, wherein the stroke length of the first piston is adjustable.
 8. The metering pump in accordance with claim 1, wherein the stroke length of the second piston is adjustable.
 9. The metering pump in accordance with claim 1, wherein the first piston and the second piston have a common longitudinal axis. 